Choi Hyoung Joon, Roundy David, Sun Hong, Cohen Marvin L, Louie Steven G
Department of Physics, University of California at Berkeley, Berkeley, California 94720, USA.
Nature. 2002 Aug 15;418(6899):758-60. doi: 10.1038/nature00898.
Magnesium diboride differs from ordinary metallic superconductors in several important ways, including the failure of conventional models to predict accurately its unusually high transition temperature, the effects of isotope substitution on the critical transition temperature, and its anomalous specific heat. A detailed examination of the energy associated with the formation of charge-carrying pairs, referred to as the 'superconducting energy gap', should clarify why MgB(2) is different. Some early experimental studies have indicated that MgB(2) has multiple gaps, but past theoretical studies have not explained from first principles the origin of these gaps and their effects. Here we report an ab initio calculation of the superconducting gaps in MgB(2) and their effects on measurable quantities. An important feature is that the electronic states dominated by orbitals in the boron plane couple strongly to specific phonon modes, making pair formation favourable. This explains the high transition temperature, the anomalous structure in the specific heat, and the existence of multiple gaps in this material. Our analysis suggests comparable or higher transition temperatures may result in layered materials based on B, C and N with partially filled planar orbitals.
二硼化镁在几个重要方面不同于普通金属超导体，包括传统模型无法准确预测其异常高的转变温度、同位素取代对临界转变温度的影响以及其反常比热。对与形成载流子对相关的能量（即所谓的“超导能隙”）进行详细研究，应能阐明为什么MgB₂与众不同。一些早期实验研究表明MgB₂有多个能隙，但过去的理论研究并未从第一性原理解释这些能隙的起源及其影响。在此，我们报告对MgB₂中超导能隙及其对可测量量影响的从头计算。一个重要特征是，由硼平面中的轨道主导的电子态与特定声子模式强烈耦合，使得对的形成变得有利。这解释了该材料的高转变温度、比热中的反常结构以及多个能隙的存在。我们的分析表明，基于硼、碳和氮且具有部分填充平面轨道的层状材料可能会有相当或更高的转变温度。
